To suppress the noise in the speed feedback signals in elevator drives, low-pass filters are currently used. However, these cause a delay in the processing of the primary signal and thus make the servo system more difficult to control. Also, non-linear median filters have been used for the filtering of a feedback signal measured by means of a tachometer generator, but they, too, produce an undesirable additional delay in the control loop. The noise appearing in tachometer signals is dealt with e.g. in an article by S.J. Ovaska, entitled "Multistage digital prefiltering of noise tachometer signals", IEEE Transactions on Instrumentation and Measurement, vol. 37, no. 3, September 1988, pp. 466-468.